The present invention relates generally to the art of heating articles and in particular to heating method and apparatus having particular application for heating eyeglass frames to permit the insertion and removal of lenses.
In the fabrication of eyeglasses, the lens are normally prepared in accordance with the patient's prescription and are thereafter inserted in the frames. In addition, lens are often replaced in a patient's existing frames. When lens are to be inserted into frames, the rims are commonly heated whereby plastic frames become deformable and metallic frames expand. While the frames are in a softened or expanded state, a lens can be snapped into place. One type of prior art apparatus for heating frames comprises a small oven. Such apparatus is not wholly satisfactory, however, because the opening and closing of the oven door makes it difficult to maintain the desired temperature. Another prior art apparatus for heating such frames is to employ a container of particulate which has a heating element disposed therebelow. This apparatus suffers the disadvantage of having substantial gradients from the lower portions of the container to its upper end. Accordingly, it is extremely difficult to obtain uniform heating. As a result, it is common to damage the frames and/or the lens because insertion too deeply into the bed causes overheating. The latter type of apparatus is also employed for sterilizing medical instruments and supplies, but also suffers the disadvantage of non-uniform temperature gradients whereby those particles located adjacent the heater must be heated to an inordinate temperature in order to provide a sufficiently high temperature at the more remote particles.
Another type of heating apparatus is a fluidized bed which consists of a loosely packed mass of solid particles contained in an open topped vessel by a porous plate. Air at high pressure and low volume is delivered to a gas chamber below the plate. This causes the particles to circulate freely and acquire the properties of a liquid. While fluidized bed heaters provide satisfactory heating characteristics, it requires a relatively large and expensive air compressor having substantial power requirements.